Catalyst for the dehydrogenation of ethylbenzene to styrene

ABSTRACT

The present invention relates to a catalyst for the dehydrogenation of ethylbenzene to styrene and to a process for the preparation thereof. The catalyst contains iron oxide, potassium oxide and calcium oxide, preferably also cerium oxide and molybdenum oxide, and is characterised by a magnesium content (expressed as MegO) lower than 0.001 %.

STATE OF THE ART

The industrial production of styrene from ethylbenzene started in theForties, but a large scale production started only in the Fifties, bymeans of a catalyst based on Fe₂O₃—Cr₂O₃—K₂CO₃, calcined at very hightemperature, usually higher than 900° C. (U.S. Pat. No. 2,461,147). Dueto the high calcination temperature; this catalyst, known with the tradename Shell 105, was characterised by high mechanical strength, but hadlow selectivity to styrene. A dramatic selectivity improvement wasachieved with catalysts based on Fe₂O₃—K₂CO₃—MoO₃—CeO₂ (U.S. Pat. No.3,904,552), which required a much lower calcination temperature (ca.500° C.). In order to confer mechanical strength to these catalysts itwas necessary to add Portland cement, which contained other elements,whose characteristics were detrimental to the catalytic process. Morerecently (U.S. Pat. No. 4,467,046), it has been found that the additionof a calcium compound (oxide or carbonate) to the formulation disclosedin U.S. Pat. No. 3,904,552, allowed to avoid the addition of Portlandcement and its negative effects. Nevertheless, the addition of calciumoxide and carbonate renders the catalysts relatively fragile.

DESCRIPTION OF THE INVENTION

It has now surprisingly been found that catalysts for thedehydrogenation of ethylbenzene to styrene which contain calcium oxideor carbonate are mechanically more resistant, more active and moreselective when the magnesium content of the calcium oxide precursor andof the resulting catalyst is particularly low.

The present invention provides a catalyst containing iron oxide,potassium oxide and calcium oxide, preferably also cerium oxide andmolybdenum oxide, characterised in that the magnesium content is lowerthan 0.001%. Preferably, the catalyst comprises (expressed as weight %of oxides): 50-95% iron oxide (Fe₂O₃), 5-20% potassium oxide, 0.2-14%calcium oxide, 0.5-20% cerium oxide and 0.2-8% molybdenum oxide. A morepreferred composition comprises: 60-85% iron oxide (Fe₂O₃), 5-15%potassium oxide, 0.5-2% calcium oxide, 5-15% cerium oxide, 0.5-2%molybdenum oxide. A most preferred composition comprises: 75-80% ironoxide (Fe₂O₃), 8-12% potassium oxide, 0.7-1.4% calcium oxide, 8-12%cerium oxide, 0.7-1.4% molybdenum oxide.

The catalyst of the invention can be prepared from iron, calcium,potassium, cerium and molybdenum salts, hydroxides or variously hydratedoxides, easily available on the market, the magnesium content of saidcompounds being such that the final catalyst contains less than 0.001%by weight of magnesium (expressed as magnesium oxide).

A further object of the present invention is therefore a process for thepreparation of the catalyst comprising the following steps:

-   -   pre-mixing iron, calcium, potassium, cerium and molybdenum        salts, hydroxides or variously hydrated oxides having a        magnesium content such that the final catalyst contains less        than 0.001% by weight of magnesium (expressed as magnesium        oxide);    -   adding a solution or a suspension of a binder-lubricating agent        so as to obtain a paste;    -   extruding a paste in cylindrical particles of diameter and        length ranging from 2 to 6 mm;    -   submitting said cylindrical particles to drying and calcination        characterised in that calcination is carried out for a time        range from 30 minutes to 10 hours, preferably from 1 to 4 hours,        at a temperature ranging from 950 to 990° C., preferably from        960 to 980° C. and more    -   preferably from 965 to 975° C.

Pre-mixing can be carried out by means of conventional techniques, suchas dry or wet milling, e.g. in a ball mill or in another suitableapparatus and added with a proper amount of a solution or suspension ofa binder-lubricating agent, such as stearic acid,carboxymethyl-cellulose, polyethylene glycol, glycerol, starch ofvarious origin and similar compounds, so as to form a paste of theproper consistency, suitable for the extrusion in cylindrical particles.Before calcination, the extruded cylinders are dried at a temperaturebetween 50 and 120° C., preferably between 60 and 100° C.

The catalyst of the invention can be conveniently used for thedehydrogenation of ethylbenzene to styrene by passing a flow ofethylbenzene and water vapour through a bed of catalyst particles, witha “steam/oil” (S/O) water/ethylbenzene weight ratio between 2.5 and 1.0,preferably lower than 2. It is well known that the lower the S/O ratio,the lower the energy consumption of the process, but the higher the riskof catalyst deactivation, due to deposition of carbonaceous compounds. Anitrogen flow is added to these two reactants, so as to give avolumetric dilution ratio (RD) of the gaseous flows(ethylbenzene+water)/(ethylbenzene+water+nitrogen) between 0.1 and 1,preferably between 0.4 and 0.6.

The present invention will be now illustrated in further detail by meansof some examples.

EXAMPLES Examples 1-5

A mixture of finely powdered iron oxide, cerium carbonate, calciumcarbonate, potassium carbonate and potassium molybdate, in proper weightratios, (said mixture containing less than 0.001% by weight ofmagnesium, expressed as magnesium oxide) has been dry milled for 4 hoursin a corundum ball mill, so as to give a final catalyst with thefollowing weight % composition: Fe₂O₃ 78, CeO₂ 10, CaO 1, K₂O 10,MoO₃ 1. A 5% by weight aqueous suspension of carboxymethyl-cellulose asbinder-lubricating agent has been then added to the mixture so as toform a paste having a consistency suitable for extrusion through a diewith holes 3 mm in diameter. The extrudates have been dried overnight at80° C. in an oven and divided in 5 portions. The latter have beencalcined separately in flowing air, with a temperature ramp of 1°C./min, up to 900, 950, 970, 990 and 1000° C., respectively. The finalcalcination temperature has been maintained for 3 hours and then thesamples have been allowed to cool down to room temperature. Thecylindrical particles thus obtained (3×3 mm in size), have beensubjected to the crushing strength essay, as described in the literature(see e.g. J. T. Richardson, “Principal of catalyst development”, PlenumPress, New York 1989) and then crushed and sieved, and the 40-60 meshfraction (samples IC1-90, IC1-95, IC1-97, IC1-99 and IC1-100) has beenrecovered.

Every sample has been tested by loading 1 g of catalyst in a continuoustubular laboratory reactor, made of Incoloy 800 alloy (internal diameter9 mm, with an axial thermo well of 1.6 mm external diameter). A nitrogenflow has been then fed to the reactor and the temperature was raised by3.17° C./min up to 400° C., then by 1.75° C./min up to 610° C., thenmaintained. The feeding of water and of ethylbenzene was started at 300°C. and at 550° C., respectively. The space velocity (LHSV) was 0.7 cm³of ethylbenzene/(hour×cm³ of catalyst bed), the S/O ratio was 2 and theRD ratio was 0.5.

The samples of reactor effluent for the determination of the activityand selectivity of the catalyst, have been collected by means of traps,cooled to −40° C. by a cryostat. The conversion of ethylbenzene and theselectivity to styrene have been determined at 48 hours-on-stream, bygas-chromatographic analysis of the effluent samples collected in thetraps. The results of the mechanical strength and catalytic activityassays are reported in table 1.

Example 6

A sample of catalyst, referred to as IC3-97a, consisting of: Fe₂O₃ 78%,CeO₂ 10%, CaO 1%, K₂O 10%, MoO₃ 1%, prepared by employing as CaOprecursor a commercial calcium carbonate containing 0.5% by weight ofmagnesium carbonate, and operating as described in examples 1-5, hasbeen calcined at 970° C. and tested under the conditions describedabove. The results are reported in Table 1.

Example 7

A sample of catalyst, referred to as IC3-97b, consisting of: Fe₂O₃ 77%,CeO₂ 10%, CaO 1%, K₂O 10%, MoO₃ 1%, MgO 1%, has been calcined at 970° C.and tested under the conditions described above. The results arereported in Table 1.

Example 8

A sample of catalyst IC1-97, tested under the same conditions as thoseof examples 1-5, but with a S/O ratio of 1.5, gave the results reportedin Table 1. These results remained practically unaltered after 900 hourson stream. TABLE 1 Conv. of Ethylbenzene Selectivity to Mech. CatalystS/O Mol % Styrene Mol % Strength IC1-90 2 69.26 93.06 fair IC1-95 275.30 93.37 good IC1-97 2 85.53 94.04 excellent IC1-99 2 77.40 94.01excellent IC1-100 2 65.35 94.68 excellent IC3-97a 2 73.42 94.58 fairIC3-97b 2 66.04 94.60 insufficient IC1-97 1.5 84.50 94.02 excellent

The above examples demonstrate that other conditions being the same(calcination temperature, nature of precursors and preparationprocedure) the presence of magnesium noticeably lowers both mechanicalstrength and conversion, while insignificantly affects selectivity(compare catalyst IC1-97, virtually magnesium-free, with IC3-97a,containing significant traces of MgO, and with IC3-97b, containing 1% ofMgO).

For identical compositions, the higher the calcination temperature, thehigher the mechanical strength of the catalyst (compare catalystsIC1-90, IC1-95, IC1-97, IC1-99 and IC1-100). When the calcinationtemperature exceeds 970° C., a considerable decrease of activity can beobserved, the decrease being remarkable when the calcination temperatureexceeds 990° C. (compare catalysts IC1-97, IC1-99 and IC1-100).

1. A catalyst for the dehydrogenation of ethylbenzene to styrene,containing iron oxide, potassium oxide and calcium oxide, characterisedin that the magnesium content expressed as magnesium oxide is lower than0.001%.
 2. A catalyst according to claim 1 characterised in that it alsocontains cerium oxide and molybdenum oxide.
 3. A catalyst according toclaim 2, comprising: 50-95% Fe₂O₃, 5-20% K₂O, 0.2-14% CaO, 0.5-20% CeO₂,0.2-8 MoO₃.
 4. A catalyst according to claim 3, comprising: 60-85%Fe₂O₃, 5-15% K₂O, 0.5-2% CaO, 5-15% CeO₂, 0.5-2 MoO₃.
 5. A catalystaccording to claim 4, comprising: 75-80% Fe₂O₃, 8-12% K₂O, 0.7-1.4% CaO,8-12% CeO₂, 0.7-1.4 MoO₃.
 6. A catalyst according to claim 5,comprising: 78% Fe₂O₃, 10% K₂O, 1% CaO, 10% CeO₂, 1% MoO₃.
 7. A processfor the preparation of the catalyst of claim 1, comprising: pre-mixingiron, calcium, potassium, cerium and molybdenum salts, hydroxides orvariously hydrated oxides; adding a solution or a suspension of abinder-lubricating agent so as to obtain a paste; extruding the paste incylindrical particles of diameter and length ranging from 2 to 6 mm;submitting said cylindrical particles to drying and calcinationcharacterised in that the catalyst contains less that 0.001% ofmagnesium, expressed as magnesium oxide.
 8. A process according to claim7, characterised in that the final calcination of the catalyst iscarried out in a temperature range from 950 to 990° C.
 9. A processaccording to claim 8, characterised in that the final calcination iscarried out in a temperature range from 960 to 980° C.
 10. A processaccording to claim 9, characterised in that the final calcination iscarried out in a temperature range from 965 to 975° C.
 11. A processaccording,to claim 10, characterised in that the final calcination iscarried out at 970° C.
 12. A process according to claim 6, characterisedin that the final calcination time ranges from 30 min to 10 hours.
 13. Aprocess according to claim 12, characterised in that the finalcalcination time ranges from 1 hour to 4 hours.
 14. A catalystobtainable according to the process of claim
 7. 15. A process for thedehydrogenation of ethylbenzene to styrene which comprises passing aflow of ethylbenzene and water vapour through a bed of particles of thecatalyst of claim
 1. 16. A process according to claim 15, characterisedin that the water/ethylbenzene ratio is lower than
 2. 17. A processaccording to claim 16, characterised in that the water/ethylbenzeneratio not higher than 1.5.